For the production of wells for the purpose of recovering hydrocarbons and other fluid and/or gaseous materials, boreholes are drilled into subterranean formations. Typically, a fluid is circulated during drilling from the surface through the interior of the drill string and the annulus between the drill string and the formation and back.
Drilling fluids, also referred to as “drilling muds”, which include water-based and organic-based drilling fluids, such as oil-based and so-called synthetic-based drilling fluids, are used in the drilling of such wells. Drilling fluids typically have a number of functions including: lubricating the drilling tool and drill pipe which carries the tool; suspending and transporting formation cuttings to the surface for screening out and disposal; holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid; and counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues. Having regard to these functions generally the drilling fluid used for a particular drilling operation is typically designed or otherwise selected to have particular rheological and density properties.
For a drilling fluid to perform the aforesaid functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered during drilling in which small or substantial amounts of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation, which may be either pre-existing or drilling-induced, or through a porous formation surrounding the borehole.
The basic mechanism of fluid loss control generally involves the formation of a filter cake of varying thickness, during drilling, on the sides of the borehole and hence at the interface of the porous or permeable formation. Unfortunately, subterranean formation conditions are frequently encountered during drilling which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
In these instances, fluid-loss control agents are added to the drilling fluid in an attempt to reduce the unacceptable high losses of drilling fluid to fractures and/or porous structure in the surrounding formation. Known examples of such fluid-loss control agents include graphitic carbon or graphite particles (see e.g. U.S. Pat. No. 5,826,669 (Zakeski) or WO 04/092301 (publication date Oct. 28, 2004)), or water soluble polymeric celluloses, starches or other polysaccharide derivatives. Other fluid-loss control agents have been described including gilsonite mineral powder (see e.g. Society of Petroleum Engineers, paper no SPE 77446 (Aston et al.)); crumb rubber particles (see e.g. U.S. Pat. No. 6,518,224 (Wood)); oil soluble silicone resin particles (see e.g. WO 04/013251 (publication date Feb. 5, 2004)); water insoluble polymer latices (see e.g. U.S. Pat. No. 6,715,568 (Bailey)), organosoluble polymer (see e.g. Society of Petroleum Engineers, paper nos. SPE 64981 (Herzhaft et al.) and SPE 107281 (Guichard et al.)), particulate polyolefin (see e.g. U.S. Pat. No. 4,436,636 (Carnicom)) and polymeric polypropylene granules (see e.g. WO 06/040578 (publication date Apr. 20, 2006).